Liquid crystal display elements are display element having a thin thickness, light weight, and low power consumption among various display elements. This leads to wide application of liquid crystal display elements such as image display apparatus such as television, various monitors, and OA (Office Automation) apparatuses such as personal computers.
Conventionally known liquid crystal display methods of the liquid crystal display elements include, for example, a TN (Twisted Nematic) mode in which a nematic liquid crystal is used, display modes in which FLC (Ferroelectric Liquid crystal) or AFLC (Antiferroelectric Liquid crystal) is used, IPS (In-plane switching) mode, FFS (Fringe Field switching) mode (see later-described Patent Literature 1), etc.
Among the liquid crystal display methods, for example, the TN (Twisted Nematic) mode in which the nematic liquid crystal is used is conventionally adopted in the liquid crystal display elements in practical use. The liquid crystal display elements using the TN mode have disadvantages of slow response, narrow viewing angle, and other drawbacks.
Moreover, the display modes in which the FLC or AFLC is used, are advantageous in their fast response and wide viewing angles, but significantly poor in anti-shock property and temperature characteristics. Therefore, the display modes in which the FLC or AFLC is used have not been widely used.
Moreover, the IPS mode and FFS mode performs displaying by switching liquid crystal molecules in plane and advantageously have wide viewing angle. Like the TN mode, however, the IPS mode and FFS mode are disadvantageous in slow response. Those disadvantages are large hindrances for the IPS mode and FFS mode to take over CRT (Cathode Ray Tube).
In all the foregoing liquid crystal display methods, liquid crystal molecules are oriented in a certain direction and thus a displayed image looks differently depending on an angle between a line of vision and the liquid crystal molecules. On this account, all these display methods have viewing angle limits. Moreover, all the display methods utilize rotation of the liquid crystal molecules, the rotation caused by application of an electric field on the liquid crystal molecules. Because the liquid crystal molecules are rotated in alignment all together, responses take time in all the display method. Liquid crystal display elements using the display mode in which the FLC or the AFLC is used are advantageous in terms of response speed and viewing angle, but have a problem in that their alignment can be irreversibly destroyed by an external force.
In contrast to those liquid crystal display elements in which rotation of molecules by the application of the electric field is utilized, a liquid crystal display element in which a material having an optical anisotropy by and according to electric field application is used is proposed. Especially, a liquid crystal display element in which a material showing an orientation polarization due to electro-optical effects is utilized, and a liquid crystal display element in which a material showing electronic polarization is used.
The electro-optical effect is a phenomenon in which a refractive index of a material is changed by an external electric field. There are two types of electro-optical effect: one is an effect proportional to the electric field and the other is proportional to the square of the electric field. The former is called the Pockels effect: the latter is called the Kerr effect. The Kerr effect was adopted early on in high-speed optical shutters, and has been practically used in special measurement instruments. The Kerr effect was discovered by J. Kerr in 1875. So far, organic liquid such as nitrobenzene, carbon disulfide, and the like, are known as material showing the Kerr effect. These materials are used, for example, in the aforementioned optical shutters, and the similar devices. Further, these materials are used, e.g. for measuring strength of high electric fields for power cables and the like, and similar usage.
Later on, it was found that liquid crystal materials have large Kerr constants. Researches have been conducted to utilize the large Kerr constant of the liquid crystal materials for use in light modulation devices, light deflection devices, and further optical integrated circuit. It has been reported that some liquid crystal compound has a Kerr constant more than 200 times higher than that of nitrobenzene.
Under these circumstances, studies for using the Kerr effect in display apparatuses have begun. The refractive index of the materials showing the Kerr effect is proportional to square of the electric field applied on the material. Because of this, a relatively lower voltage driving is expected in the utilization of orientation polarization with a material showing the Kerr effect than in the utilization of orientation polarization with a material showing the Pockels effect. Further, it is expected that the utilization of a material showing the Kerr effect attains a high-speed response display apparatus because, e.g., the Kerr effect shows a response property of several p seconds to several m seconds, as its basic nature.
Under there circumstances, for instance, Patent Literatures 2 and 3 suggest display elements in which a medium made from a liquid crystalline material is sealed between a pair of substrates and the Kerr effect is induced by application of an electric field parallel to the substrates.
In the display elements disclosed in Patent Literatures 2 and 3, the electric field cannot affect the medium in regions on electrodes (i.e., regions superimposing the electrodes in the normal direction of the plane of the substrates). Thus, the display elements disclosed in Patent Literatures 2 and 3 have a problem of a being less transmissive due to these regions, which cannot be utilized as display regions.
In the technologies of Patent Literatures 2 and 3, an electric field substantially parallel to the plane of the substrates is formed by using pairs of electrodes formed in the plane of the substrates. In this arrangement, the medium is not affected in the regions on the electrodes by the electric field, whereby the medium on the regions on the electrodes maintains initial orientation (orientation attained when no electric field is applied). This makes it impossible to utilize the region on the electrodes as the display region. Thus, a display panel becomes less transmissive due to this. As a result, it is necessary to use a backlight of a high brightness in order to attain appropriate brightness, thereby resulting in higher power consumption.
Patent Literature 1 discloses a liquid crystal display device in which all liquid crystal molecules existing in regions on electrodes are movable with such a configuration that (i) counter electrodes and pixel electrodes are made of a transparent material, (ii) the counter electrodes and the pixel electrodes are arranged with an electrode-to-electrode distance smaller than a cell gap so that a large number of fringe electric fields occur, and (iii) driving electrodes have a width narrow enough to move all the liquid crystal molecules by the fringe electric fields occurred on both sides of the respective driving electrodes. However, the liquid crystal display device of Patent Literature 1 uses a conventional generally-used liquid crystal material, that is, a liquid crystal material whose liquid crystal molecules, which are aligned in a rubbing direction of an alignment film when no electric field is applied, are rotated together in alignment by electric field application. Thus, the problem of slow response speed is left unsolved in the liquid crystal display device of Patent Literature 1.
Moreover, the alignment film that has been subjected to rubbing process is necessary in the technology of Patent Literature 1. In the rubbing process, the alignment film made of a polymer such as polyimide is rubbed with a cloth or the like. Thus, the rubbing process is associated with fine dust and fine electric discharge (local discharge) which occurs due to high voltage static electricity. The dusts are a big problem in highly fine pixel electrodes and TFT forming process in which film deposition, exposure, and etching are repeated. The local electric discharge would damage the alignment film, or cause disconnection or electrostatic discharge damage in TFT and transparent electrode such as ITO. On this account, it is desirable to omit the rubbing process that would cause such problems.
Moreover, the liquid crystal display device disclosed in Patent Literature 1 has such a problem that the orientation would be disturbed by an electric field that occurs locally in the vicinity of an electrode edge. Because orientation changes have a long correlation distance in conventional generally-used liquid crystal materials, such orientation disturbance would be spread out widely from the vicinity of the electrode edge to remote regions. This would cause deterioration in display performance.
Patent Literature 1    Publication of Japanese Patent Application, publication No. 11-202356 (Tokukaihei 11-202356 (Jul. 30, 1999)(Publication of Japanese Patent Application, publication No. 2001-249363 (Tokukai 2001-249363; published on Sep. 14, 2001))
Patent Literature 2    Publication of Japanese Patent Application, publication No. 11-183937 (Tokukaihei 11-183937; published on Jul. 9, 1999) corresponding to U.S. Pat. No. 6,266,109)
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